I give yogurt occasionally, but when I want to support the immune system, or when I'm giving antibiotics, I give straight probiotics - far more bang for the buck than yogurt. Most yogurts don't really have a ton of live bacteria because they're pasteurized (a process which kills bacteria); some brands (more and more these days, no doubt) will add bacteria back into the yogurt post-pasteurization. I give the liquid probiotics, from the refrigerator section of the health food store; others give capsules or powders, but I've found the liquids are better tolerated by my crew.